Chlamydia are prevalent human pathogens causing disorders such as sexually transmitted diseases, respiratory diseases including pneumonia, neonatal conjunctivitis, and blindness. Chlamydia are obligate intracellular bacteria that infect the epithelial lining of the lung, conjunctivae or genital tract. The most common species of Chlamydia include Chlamydia trachomatis, Chlamydia psittaci, Chlamydia pecorum and Chlamydia pneumoniae. Recently, the newly designated species of Chlamydia, C. pneumoniae (formerly C. trachomatis TWAR), has been implicated as a major cause of epidemic human pneumonitis and perhaps may play a role in atherosclerosis.
There are currently 18 recognized C. trachomatis serovars, causing trachoma and a broad spectrum of sexually transmitted diseases: with the A, B and C serovars being most frequently associated with trachoma, while the D-K serovars are the most common cause of genital infections.
C. trachomatis is the major cause of sexually transmitted disease in many industrialized countries, including the United States. While the exact incidence of C. trachomatis infection in the U.S. is not known, current epidemiological studies indicate that more than 4 million chlamydial infections occur each year, compared to an estimated 2 million gonococcal infections. While all racial, ethnic and socioeconomic groups are affected, the greatest prevalence of chlamydial infections occur among young, 12 to 20 year-old, sexually active individuals. Most genitourinary chlamydial infections are clinically asymptomatic. Prolonged carriage in both men and women is common. As many as 25% of men and 75% of women diagnosed as having chlamydial infections have no overt signs of infection. As a consequence, these asymptomatic individuals constitute a large reservoir that can sustain transmission of the agent within the community.
Far from being benign, serious disease can develop from these infections including: urethritis, lymphogranuloma venereum (LGV), cervicitis, and epididymitis in males. Ascending infections from the endocervix commonly gives rise to endometritis, pelvic inflammatory disease (PID) and salpingitis which can cause tubal occlusion and lead ultimately to infertility.
C. trachomatis infection of neonates results from perinatal exposure to the mother's infected cervix. Nearly 70% of neonates born vaginally to mothers with chlamydial cervicitis become infected during delivery. The mucus membranes of the eye, oropharynx, urogenital tract and rectum are the primary sites of infection. Chlamydial conjunctivitis has become the most common form of ophthalmia neonatorum. Approximately 20-30% of exposed infants develop inclusion conjunctivitis within 14 days of delivery even after receiving prophylaxis with either silver nitrate or antibiotic ointment. C. trachomatis is also the leading cause of infant pneumonia in the United States. Nearly 10-20% of neonates delivered through an infected cervix will develop chlamydial pneumonia and require some type of medical intervention.
In developing countries, ocular infections of C. trachomatis cause trachoma, a chronic follicular conjunctivitis where repeated scar formation leads to distortion of the eyelids and eventual loss of sight. Trachoma is the world's leading cause of preventable blindness. The World Health Organization estimates that over 500 million people worldwide, including about 150 million children, currently suffer from active trachoma and over 6 million people have been blinded by this disease.
In industrialized countries, the costs associated with treating chlamydial infections are enormous. In the U.S., the annual cost of treating these diseases was estimated at $2.5-3 billion in 1992 and has been projected to exceed $8 billion by the year 2000.
One potential solution to this health crisis would be an effective chlamydial vaccine. Several lines of evidence suggest that developing an effective vaccine is feasible.
Studies in both humans and primates have shown that short-term protective immunity to C. trachomatis can be produced by vaccinating with whole Chlamydia. However, protection was characterized as short lived, serovar specific, and due to mucosal antibody. Additionally, in some vaccines disease was exacerbated when these individuals became naturally infected with a serovar different from that used for immunization. This adverse reaction was ultimately demonstrated to be due to a delayed-type hypersensitivity response. Thus, the need exists to develop a subunit-based chlamydial vaccine capable of producing an efficacious but nonsensitizing immune response. Such a subunit vaccine may need to elicit both mucosal neutralizing secretory IgA antibody and/or cellular immune response to be efficacious.
Subunit vaccine development efforts to date have focused almost exclusively on the major outer membrane protein (MOMP). MOMP is an integral membrane protein of approximately 40 kDa in size and comprises up to about 60% of the infectious elementary body (EB) membrane protein (Caldwell, H. D., J. Kromhout, and L. Schachter. 1981. Infect. Immun., 31:1161-1176). MOMP imparts structural integrity to the extracellular EB and is thought to function as a porin-like molecule when the organism is growing intracellularly and is metabolically active. With the exception of four surface exposed variable domains (VDI-VDIV), MOMP is highly conserved among all 18 serovars. MOMP is highly immunogenic and can elicit a local neutralizing anti-Chlamydia antibody. However, problems exists with this approach.
To date, most MOMP-specific neutralizing epitopes that have been mapped are located within the VD regions and thus give rise only to serovar-specific antibody. Attempts to combine serovar-specific epitopes in various vaccine vectors (e.g. poliovirus) to generate broadly cross-reactive neutralizing antibodies have been only marginally successful (Murdin, A. D., H. Su, D. S. Manning, M. H. Klein, M. J. Parnell, and H. D. Caldwell. 1993. Infect. Immun., 61:4406-4414; Murdin, A. D., H. Su, M. H. Klein, and H. D. Caldwell. 1995. Infect. Immun., 63:1116-1121).
Two other major outer membrane proteins in C. trachomatis, the 60 kDa and 12 kDa cysteine-rich proteins, as well as the surface-exposed lipopolysaccharide, are highly immunogenic but, unlike MOMP, have not been shown to induce a neutralizing antibody (Cerrone et al., 1991, Infect. Immun., 59:79-90). Therefore, there remains a need for a novel subunit-based chlamydial vaccine.